[OkayPhysicist]

It does, though. Because it's massless, it either needs to be going at max speed or zero speed. And a zero-mass, zero-energy object is a pretty good working definition for "nothing", so photons must travel at the speed of causality, thus making it "the speed of light".

[flowerthoughts]

Thanks for the reply. That's still a theoretical reasoning. "Based on our current _models_, it must follow that c=c'." I can accept that. I guess part of a wider theoretical answer is that a photon is just an interaction in quantum fields, and that indicates there's nothing special about a photon that could limit its speed (as you imply.) What you're saying makes me think I should be looking for impediments for attaining speed, and it seems only (inertial) mass is that thing.

My question is if this part of the model has been validated experimentally somehow.

BTW, it seems odd calling a photon a zero-energy object.

[OkayPhysicist]

Photons are zero-mass, some energy. (and thus moving at max speed). I was trying to convey that the only way for something to be massless and not moving at the speed of light would be for the massless object to be stationary (and thus zero energy).